1. Field of the Invention
This invention relates to improvements in or relating to an electric contact accommodated in an enclosed case of a switching apparatus, and more particularly to a switching apparatus which includes an electric contact from which a harmful component which brings about a contacting problem of the contact may possibly be produced and a method of suppressing activation of an electric contact of the switching apparatus of the type mentioned.
2. Description of the Related Art
In an electric switching apparatus such as a relay or a switch wherein electric contacts are accommodated in an enclosed case, if organic compounds are generated in the enclosed case, then this forms black nonconductor called black powder which brings about a contacting problem of the contacts and the contacts are activated in which arc discharge is likely to occur therebetween as a result of chemical reaction of organic compounds. Therefore, formation of black powder makes a serious problem particularly with a miniature relay for communication and like apparatus.
Several countermeasures for preventing an injurious effect by black powder are known and disclosed, for example, in the following documents.
1 Japanese Patent Laid-Open Application No. Heisei 6-162859
From the point of view that organic gas is attracted to the surfaces of contacts and carbonized to form black powder, which increases the contact resistance, in order to prevent such attraction and carbonization of organic gases, a physical protective film of organic compounds having a comparatively low vapor pressure such as polyhydric alcohol is positively formed on the surfaces of the contacts. The protective film exhibit additional physical actions of attraction of corrosive gas and suppression of abrasion of the surfaces of the contacts.
2 Japanese Patent Laid-Open Application No. Showa 63-80738
Electric contacts built in and used with a small motor or the like are placed in an ether and alcohol atmosphere in a case so as to form, on the contacts, a film which provides lubricity to the surfaces of the contacts to reduce the contact resistance between and prevent abrasion of the contacts.
However, the conventional techniques described above have the following problems.
The first provides resides in that, where the contact load is low, the contact resistance is unstable, which gives rise to a new problem that the contacting stability is lost. The reason is that both of the conventional techniques are intended, in order to suppress production of black powder and increase in contact resistance, to physically prevent attraction of organic gases to the surfaces of contacts by attracting organic compounds containing oxygen to the surfaces of the contacts to artificially form an organic film on the surface of the contacts (organic compounds containing oxygen are used by a large amount so that the organic compounds containing oxygen may form an organic film of a sufficient thickness on the surfaces of the contacts).
The second problem resides in that it is difficult to continue to prevent attraction of organic gases to the surfaces of contacts for a long period of time. The reason is that, since organic gas itself which may possibly make black powder is not reduced, the density of the organic gases increase as time passes.
The third problem resides in that, once a contact becomes activated, the deteriorating tendency of production of black powder cannot be suppressed any more. The reason is that attraction of organic gas to the surfaces of contacts is merely prevented physically by an organic film and the reaction itself of the surfaces of the contacts is not controlled.
By the way, chemical reactions on the surface of a contact are roughly divided into the following two reactions.
The first reaction is a reaction when contacts are opened and closed, in which black powder produced by a chemical reaction caused by arc discharge which occurs between the contact. Arc is enhanced by the presence of black powder and accelerates the formation of black powder, which result in the increase in contact resistance.
The second reaction is a reaction when no current flows between contacts, in which the surfaces of the contacts are put into a chemically active condition by friction upon opening and closing of the contacts or exposure of fresh surfaces of the contact materials and conversion of organic compounds, which is present in a case of the apparatus, into black powder is accelerated by a tribochemical reaction originating from a chemical activity of the surfaces of the contacts.